If a strong electric field is applied to a metal surface, a potential barrier has an inclination at a boundary to vacuum. If the electric field becomes at least 107 V/cm, the barrier becomes superfine-pointed and electrons are emitted into the vacuum. This is called field emission. Dispersion of energy of emitted electrons becomes small and it is approximately 0.3 eV. An apex of a field-emission electron source is finished to have a radius of curvature of approximately 100 nm in order to generate a strong electric field.
Since the size of the spot diameter of the field-emission electron gun is as small as 5 to 10 nm, the field-emission electron gun has a feature that the brightness is extremely high and it is used often as an electron gun for high resolution SEMs and TEMs. Furthermore, since the energy width of emitted electrons is small, it is easy to obtain a high resolution even at a low acceleration voltage.
On the other hand, since it operates at the room temperature, the emission current is apt to become unstable due to gas adsorption. Ultra-high vacuum is needed. There is a possibility that the cathode surface will become rough due to shocks of residual gas molecules which are ionized by emitted electrons and the cathode will be destroyed finally. As described in Patent Literature 1, therefore, instantaneous heating of the cathode called flashing is conducted sometimes to remove adsorbed gas.
As the field-emission electron source, a needle (tip) of tungsten (W) is usually used. If flashing is conducted on the W electron source at a temperature of at least 1,500 K, the adsorbed layer is evaporated and a clean surface is obtained.
If the W surface is clean, electrons are emitted mainly from a (111) plane and a (310) plane which are relatively low in work function among all planes as indicated by a field emission pattern shown in FIG. 2. As the field-emission electrode source, therefore, a W<111> or a W<310> field-emission electron source having the (111) plane or the (310) plane disposed on the apex is used.